A new teacher, Yamaguchi Kumiko, came to Shirokin and as it turns out, she's the homeroom teacher of a class filled with delinquents who tried to scare her off. However, it's not going to be easy to scare off this teacher because she's actually from the Oedo family, a yakuza family, and her grandfather's the head of the family. Trouble arises when one of Yamaguchi's students, Kumai, is accused of stealing money from the school. As it turns out, he did and Yamaguchi's here to prevent Kumai from getting expelled.

The class gave Yamaguchi a nickname: Yankumi. Minami is dating a girl, but troubles arise when it turns out that the girl was seeing another man in a rival school! When Minami was in trouble and his classmates didn't help, Yankumi scolded at the class for abandoning those they claimed were there "friends". However, Shin helped him out.

A thief has been going around stealing purses from women and the Head Teacher has been accusing Uchiyama of the crime. Yankumi and a few students decide to capture the real criminal and clear Uchiyama's name.

The school found out that Shizuka, the English Teacher, was a part-time hostess, and before the Head Teacher could fire her, a deal was made: all students from class 3-D must pass their English midterm with at least 30 points. However, with a class filled with delinquents who were lacking in academically, this might not be as easy as it seems.

Sawada Shin found out about Yankumi's family. A gang of highscool students have been attacking people with metal poles while Kumai falls in love with a girl named Ami and protects her from that gang. Yankumi teach the students the difference between violence and fighting.

It's the sports competition festival. After a deal with the Head Teacher, the students of 3-D were excited to get first place in a sport, but they didn't have enough people until Yankumi realied something: Class 3-D actually has another student in the class! Turns out that the student was a victim of bullying and couldn't bare to go back to school. With determination, Yankumi does everything to persuade the student to take part in the competition.

Everyone found out that Sawada Shin has a younger sister and Shin actually lives alone after bickering with his father. Shin's sister, under a lot of pressure from their parents, decided to go to a dangerous club. Shin found out and tried to help her sister escape, but after police officers came and Shin was determined to make sure his sister wasn't caught, Shin was the one who got arrested. However, the police demands that to release Shin, one of his parents or legal guardians must pick him up.

Kawashima, the school nurse, has to go to a training camp just when her son, Yuta, comes over to see his mother. Because she can't cancel the training camp, Yankumi invites Yuta over for the night and in the morning, until Kawashima came back, Yankumi has Yuta over to her classroom. Little did they know that Yuta didn't inform his grandparents, whom Yuta lives with, that he's visiting Kawashima and that Yuta isn't actually related to Kawashima by blood. Refusing to give up, Yuta runs away and everyone hurries to find him. In the end, Yuta can live with Kawashima.

With the Shirokin volleyball team advancing towards nationals, Yankumi decided to form a cheerleading team. However, the event didn't go smoothly because of Kurosaki. Kurosaki holds a grudge against Shirokin because his volleyball coach made him quit. Kurosaki was involved in a violent incident and normally, Kurosaki would get off with a suspension, but the Coach made him quit to preserve the reputation of the team and the school. With such a strong grudge, Kurosaki is out for revenge.

When the Head Teacher's wife sets up an omiai for Yankumi, Yankumi reluctantly goes, knowing that she only has eyes for Shinohara the detective, whose sister is getting married. Later on, while Kuma is taking a stroll, he bumps into a student from a prestigious high school who was shoplifting. Kuma was blamed for the crime and Yankumi, Shin, Uchi, Minami, Noda and Kuma heads to bring the real perpetrator out.

After some students from class 3-D apprehended a serial murderer-robber, some reporters wanted a 24-hour close-up of how Yankumi's life is. However, things get out of hand when the reporters realize where Yankumi lives. Also, Kuma has a falling out with his father.

Yankumi's secret is finally revealed. She was forced to resign, but the students of class 3-D couldn't stand for it. Many students voiced out to the superintendent how Yankumi was able to change all of them and has taught them many great lessons in life, threatening to drop out of school if they don't reinstate her. In the end, when Yankumi manages to convince all her student to stay in school, Sawatari, the vice-principal, comes to the conclusion that Yankumi is a needed teacher in the school.